Question: Which of these numbers is prime? ${17,\ 27,\ 40,\ 51,\ 85}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 17 are 1 and 17. The factors of 27 are 1, 3, 9, and 27. The factors of 40 are 1, 2, 4, 5, 8, 10, 20, and 40. The factors of 51 are 1, 3, 17, and 51. The factors of 85 are 1, 5, 17, and 85. Thus, 17 is a prime number.